As a heat-shrinkable film, particularly a heat-shrinkable film for a label on a bottle body part, films including polyvinyl chloride, polystyrene, or the like are mainly used. However, with respect to polyvinyl chloride, in these years, chlorine gas generation at the time of incineration for disposal becomes a problem and with respect to polystyrene, there is a problem that printing is difficult, and heat-shrinkable polyester films almost free from these problems have drawn attention. However, the specific gravity of a polyester film is as heavy as about 1.4. Since general heat-shrinkable polyester films are transparent, the light transmittance is high, so that such films are not suitable for protection of contents. Thus, there is a great demand for a void-containing heat-shrinkable polyester film in which formation of voids reduces the specific gravity and decreases the light transmittance.
Therefore, a void-containing heat-shrinkable polyester film has been investigated. A method in which an incompatible thermoplastic resin is mixed in the main raw material of the heat-shrinkable polyester film is proposed (Patent Document 1).